The present invention is directed to phase-locked loops and other circuitry for generating an output signal that has a predetermined phase relationship with an input reference. It is directed particularly to speeding the response of such circuits.
The phase-locked loop is a circuit that has many applications and is accordingly arranged in various ways. The basic purpose of a phase-locked loop is always the same, however, namely to maintain a predetermined phase relationship between its output, which is typically generated by a voltage-controlled oscillator, and a reference signal. The reference signal may be an information-bearing signal received from an outside source. It may also be a local crystal reference.
The simplest phase relationship between the reference and the output is that they have the same phase and frequency. A phase-locked loop of this type typically is used as a filter; it "locks on" to a received signal component in the expected frequency range and reproduces the phase-modulation information in the received signal without forwarding much of the accompanying noise.
A slightly more complicated phase relationship between input and output is usually encountered in phase-locked loops used as frequency synthesizers. For instance, the predetermined phase relationship may be that a subharmonic of the output frequency is to remain in phase with the reference. In this type of an arrangement, the error signal that controls the voltage-controlled oscillator is produced by comparing to the phase of the reference signal the phase of subharmonic signal produced by applying the output of the voltage-controlled oscillator to a frequency divider.
A further complication in phase relationship occurs in the sandap, which is essentially a phase-locked loop but differs from conventional phase-locked loops in that its output is a signal that repeatedly sweeps in frequency. The phase relationship is that the output starts every sweep at the same phase with respect to a constant-frequency reference signal, and the phase relationships that obtain between the output and reference signals throughout the sweep are the same for every sweep. This keeps the output frequency a linear function of time and synchronizes the output signal with other circuitry that is generated from the same reference signal.
In both sandaps and other phase-locked loops, a representation of the output signal, such as the signal itself or a subharmonic, is compared in phase with the reference signal, and the resultant error signal is filtered to produce a control signal for the voltage-controlled (and, in the case of a sandap, chirped) oscillator. This feedback results in very accurate steady-state phase tracking. However, stability considerations dictate limitations on the gain of the feedback loop, so such circuits tend not to respond quickly to sudden errors.
An object of the present invention is therefore to speed the response of sandaps and other phase-locked loops without compromising their stability.